For wiring boards used in electronic equipment, the demand for flexible wiring boards continues to increase due to their flexibility, thinness and lightweight. Flexible wiring boards are also used as a base material for semiconductor packages and for packages for liquid crystal modules. A typical construction of the flexible wiring board includes a polyimide film as an electrically insulating base material, a thin metal tiecoat, a copper seedcoat, and a layer of electrodeposited copper. The tiecoat and seedcoat layers can be applied, for example, using vacuum deposition techniques. The process involves a plasma pretreatment of the polyimide, sputter deposited tiecoat and seedcoat metals and electroplated copper. The tiecoat metal is typically either chromium or a nickel based alloy, which serves to enhance adhesion. The purpose of the copper seedcoat is to provide sufficient electrical conductivity to permit electroplating to a final copper thickness. Thereafter, the boards are processed through steps of photoimaging, etching and stripping to form fine line wiring boards.
The fine line wiring formation can be finished by either a single step etching process that involves photoimaging, etching copper and Ni/Cr alloy together and then stripping resist, or a two-step etching process that involves photoimaging, etching copper, stripping resist and thereafter etching Ni/Cr alloy.
The etching chemistries used for single step-etching have traditionally comprised cupric or ferric chloride/hydrochloric acid solutions or permanganate acid solutions. With photoresist leaching into cupric or ferric chloride/hydrochloride acid etchant, the etch rate for Ni/Cr alloy is generally slowed down. The process also has the potential to cause too much dissolution of copper. For permanganate acid etchant, Ni/Cr etch slows due to passivation by the MnO2 reaction product, and a step of “neutralization” with oxalic acid or ascorbic acid removes MnO2 is necessary to maintain good etch rates. To solve the issues resulting from single step etching, various two-step etching processes have also been developed, in which the Ni/Cr alloy is etched after the photoresist is stripped. This process needs to be selective, such that the etching solution removes the unwanted Ni/Cr alloy without attacking the copper.
Several patents for acidic chromium etching solutions have suggested these types of properties. For example, U.S. Pat. No. 2,230,156 to Carman, the subject matter of which is herein incorporated by reference in its entirety, describes a chromium etching solution containing hydrochloric acid and glycol, the glycol having more carbon atoms than hydroxyl groups and U.S. Pat. No. 2,687,345 to Murray, the subject matter of which is herein incorporated by reference in its entirety, describes a chromium etching solution containing calcium chloride and ethylene glycol. Furthermore, U.S. Pat. No. 4,160,691 to Abulafia et al., the subject matter of which is herein incorporated by reference in its entirety, describes a chromium etching solution containing hydrochloric acid and an aliphatic alcohol such as glycerin. All of the above solutions are acid chromium etching solutions, which supposedly remove chromium with little or no attack of the copper. Thus, these etchants are suitable for etching Ni/Cr alloy layers.
In addition, U.S. Pat. No. 6,841,084 to Lillie et al., the subject matter of which is herein incorporated by reference in its entirety, describes a process for etching an electrically resistive material comprised of a nickel chromium alloy for forming an embedded resistor. However, the etching rate of this solution decreases significantly as the following ratio increases: the surface area of copper-exposed to the etching solution (CsA) and the surface area of the Ni/Cr alloy exposed to the etching solution (RsA). This ratio (hereinafter referred to as the CSA/RSA ratio) can reach a value in which the etching of the Ni/Cr alloy is inhibited.
Moreover, it has been found that some Ni/Cr alloy etching solutions may dissolve surface treatments (including, but not limited to, adhesion-promoting treatments such as described in U.S. Pat. No. 6,969,557, to Matsuda et al., thermal barrier layer treatments such as described in U.S. Pat. No. 7,510,743 to Subramanian, stain proofing pretreating and resin resistant coating treatment, such as described in U.S. Pat. No. 4,915,797 to Vigezzi, et al., that are deposited on the copper surface to enhance the peel strength and shelf life of the resistive foil. This issue can be resolved, for example, by using an etching solution for etching an electrically resistive material, that include a nickel-chromium alloy, comprising hydrochloric acid and thiourea.
U.S. Pat. No. 7,285,229 to Kuriyama, the subject matter of which is herein incorporated by reference in its entirety, describes an etching method for selectively etching at least one metal selected from nickel, chromium, nickel-chromium and palladium. The etching method involves two etching solutions that contain hydrochloric acid, compounds with seven or less carbon atoms containing a sulfur atom, and thiazole. The amount of time that the metal needs to be in the etching solution is at least about 2 to 5 minutes.
Finally, International Patent Application No. WO2007/040046, the subject matter of which is herein incorporated by reference in its entirety, describes an etching solution for nickel-chromium alloy which is capable of sufficiently removing a nickel-chromium alloy layer and does not decrease in etching ability when the copper concentration in the etchant is increased during etching.
Etchants for nickel-chromium alloys are typically characterized by containing at least the following components: sulfuric or sulfonic acid, hydrochloric acid or a chlorine compound, and a nitrite.
It would be desirable to provide an improved solution for etching a nickel chromium alloy between copper circuits without detrimentally attacking copper.
To that end, the present invention provides an etching solution containing hydrochloric acid, an acid selected from the group consisting of sulfuric acid, phosphoric acid, nitric acid, sulfonic acid, sulfamic acid and combinations of one or more of the foregoing, and a sulfur compound that comprises a sulfur atom with an oxidation state between −2 and +5, including, but not limited to, compounds such as sulfites, thiosulfates and sulfides, to etch a tie coat layer or a resistive layer comprised of a nickel-chromium alloy.